The present invention relates to a method of converting laser radiation from a pump laser into another wavelength range by stimulated Raman scattering in which the laser radiation of the pump laser is conducted through a Raman medium, thus generating Stokes radiation in the other wavelength range. The invention also relates to an apparatus for implementing the method.
A method and an arrangement of this type have been described by P. Rabinowitz, A. Stein, R. Brickman and A. Kaldor in an article entitled, "Stimulated Rotational Raman Scattering From Para-H.sub.2 Pumped by a CO.sub.2 TEA Laser," in the periodical Optics Letters. Volume 3, No. 4, October, 1978, pages 147-148. Here, stimulated Raman scattering of a CO.sub.2 pump laser beam in para-H.sub.2 generates Stokes radiation in a range of 16 .mu.m, as it is required for the isotope selective excitation of UF.sub.6 molecules in a molecular laser process.
However, for an effective conversion of this pump radiation into a wavelength range of 16 .mu.m, the peak power required for the CO.sub.2 radiation is so high that the optical components employed in the laser and in the conversion apparatus are under considerable stress. Moreover, with the high peak power involved, the danger is very high that radiation induced gas breakthroughs occur in the Raman medium which prevent Raman conversion.
A method and an arrangement which permit a reduction of the peak power of the CO.sub.2 pump radiation while maintaining good conversion efficiency is disclosed by J. L. Carlsten and R. G. Wenzel in an article entitled, "Stimulated Rotational Raman Scattering in CO.sub.2 -Pumped Para-H.sub.2," in the periodical, IEEE Journal of Quantum Electronics, Volume QE-19, No. 9, September, 1983, pages 1407-1413. The reduction of the peak power is realized in that seed radiation from a seed laser is radiated into the conversion apparatus simultaneously and colinearly with the pump radiation.
However, the required tuning of the wavelength of the seed laser radiation to the Stokes radiation determined by the wavelength of the pump radiation is difficult and expensive. Moreover, it is not easy to bring the seed radiation together with the pump radiation, to synchronize it in time and to cause it to overlap in space to a satisfactory degree.
A method and an arrangement in which the seed radiation is not furnished by a seed laser but is generated by the technology of four-wave mixing is disclosed by R. L. Byer and W. R. Trutna in an article entitled "16 .mu.m Generation by CO.sub.2 -Pumped Rotational Raman Scattering in H.sub.2," in the periodical Optics Letters. Volume 3, No. 4, October, 1978, pages 144-146. In this method, the radiation of a short-wave auxiliary laser must be reflected into the conversion apparatus simultaneously and colinearly with the pump radiation. Due to the greater Raman amplification at the shorter wavelength, the radiation of the auxiliary laser is converted by Raman conversion immediately upon entering the conversion apparatus. In the presence of the pump radiation, the Stokes radiation is here also generated at the wavelength determined by the pump radiation and it then acts as a seed radiation.
The thus generated seed radiation thus automatically has the correct wavelength. But there remains the difficulty of bringing together the pump radiation and the radiation of the auxiliary laser, to synchronize them in time and to cause them to suitably overlap in space.